1. Field of the Invention
The present invention relates to devices for automatically transporting delicate articles, such as reticles, masks, or wafers.
2. Description of Prior Art
Integrated circuits are made by photolithographic processes that use photo masks (or reticles) and an associated light source to project a circuit image onto a silicon wafer. High production yields are contingent on the creation and maintenance of defect-free masks. Unfortunately, mask defects are inevitable, and must therefore be found and repaired before such defects result in the manufacture of defective integrated circuits.
Due to their extraordinary precision and complexity, masks used to produce integrated circuits are very expensive. It is therefore of paramount importance that such masks are handled with extreme care, for even the least mask defect can render a mask useless.
Some conventional systems for handling masks (e.g., systems for loading masks onto inspection stages of mask-inspection systems) rely on the diligence of a human operator for handling. Others systems use robots for some tasks; however, such systems still rely on human operators because the mask must be correctly placed onto a robot loading station. Unfortunately, people are relatively slow, quickly fatigued by repetitive tasks, and frequently introduce handling and placement errors that are potentially catastrophic. Software glitches and mechanical errors introduced by conventional robots are also a source of error. These human, mechanical, and software errors should be minimized.
Some automated mask-transfer systems are adapted to work with conventional protective cassettes that contain the masks. Such mask-transfer systems are capable of removing masks from cassettes and transferring the masks to another location (e.g., onto an inspection stage). Systems that employ cassettes still rely upon the diligence of human operators for correct mask placement. The mask must be correctly oriented within the cassette, and the cassette must be correctly placed on the loading station. In either case, the failure of the human operator to correctly position the mask can result in mask damage. And, in those systems that employ robots, software or mechanical glitches may also result in mask damage.
Semiconductor manufacturers have made it clear that mask transfer systems that damage masks are unacceptable. There is therefore a need in the art for a reliable automated mask-transfer system that ensures correct mask orientation and that transfers masks without appreciable risk of mask damage.